Improved sharpness of a drill, prevention of occurrences of burrs at the exit side of a machined bore (at that portion at which the machined bore opens to a drilled face), suppression of an enlarged amount of the bore size attributable to drilling (prevention of an excessive increase of the machined bore), assurance of strength of the drill, etc. are important in drilling.
Conventionally, to improve the sharpness of a chisel edge provided at the distal end of a drill, i.e., the sharpness of the drill, a primary or secondary face having a positive rake angle is formed in a chisel-edge-forming portion of the drill (see, U.S. Pat. No. 4,688,972). In this case, however, the thickness of the drill becomes small at the chisel-edge/rake-face-forming portion of the distal end of the drill. Thus, there occurs a drawback such that the strength and rigidity of that drill portion become insufficient.
It is considered that burrs are caused by the flow of machined material (chip) flowing in the radial direction from the inward of the machined bore to the outward thereof. Thus, it appears desirable to provide a cutting edge at a peripheral portion of the distal end of the drill, so as to cause the flow of machined material to be directed toward the inward direction. However, during the course of drill production, it is impractical to machine a drill to be produced in such a manner that the drill is simultaneously formed at its distal end with both of a chisel edge face and an outer peripheral cutting edge, because interference can occur between a tool for forming the rake face and a tool for forming the cutting edge.
To suppress the excessive enlargement of the bore size resulting from the drilling, conventionally employed is a drill having a pair of cutting edges which have the same shape and are well-balanced with each other, or a drill having a single cutting edge and a guide shoe.
Conventionally, various countermeasures have been taken for the sake of improvement of the drill sharpness, prevention of burrs, and suppression of excessive enlargement of the bore size, as mentioned above. However, in the industrial production of drills, difficulties have been encountered in adopting all the countermeasures simultaneously. Particularly, it is difficult to provide the resultant drills with an adequate strength.